Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 03005810 2018-05-17
WO 2017/087997 PCT/ZA2016/050049
1
IMPROVED ILMENITE SMELTING PROCESS
BACKGROUND OF THE INVENTION
[0001] This invention relates to a consolidated process for the preparation of
carbon-based
ilmenite pellets, the solid-state reduction thereof, and the subsequent
smelting thereof in an
electric furnace.
[0002] The smelting of ilmenite consumes substantial quantities of electrical
energy.
Additionally, the operability of the furnace can be hindered due to frothing
effects.
[0003] Feed to the smelting furnace is generally made up of raw ilmenite ore
and a solid,
carbonaceous reductant. Raw ilmenite, in a particular process, is replaced by
pre-reduced
ilmenite pellets. The steps for the production process of the latter are to
prepare ilmenite
pellets using bentonite and to pre-reduce the pellets in a rotary kiln in the
presence of a solid.
carbonaceous reductant in excess. The smelting of the pre-reduced ilmenite
pellets is thus
carried out in an AC furnace. The TiO2 slag produced in this way is, however,
contaminated
with the bentonite which is an inorganic binder.
.. [0004] An object of the present invention is to provide an alternative
process for pre-reducing
an ore essentially targeting the metallisation of iron oxides contained in the
ore.
SUMMARY OF THE INVENTION
[0005] The invention provides a method of preparing a pre-reduced ilmenite ore
for smelting,
wherein metal oxides, such as iron, chromium and manganese oxides contained in
the ore
CA 03005810 2018-05-17
WO 2017/087997 PCT/ZA2016/050049
2
are selectively reduced in solid-state reactions over titanium oxide, the
method including the
step of pre-reducing carbon-based pellets of the ore.
[0006] The metal oxides, other than titanium oxides, in the pellets may be pre-
reduced to a
maximum extent i.e. essentially fully or they may be partially pre-reduced.
[0007] The pellets may be less than 6mm in size and preferably lie in the
range of 2mm to
5mm.
[0008] The pellets may be prepared from a blend of required proportions of the
ore, coal
fines of -106 microns and a suitable organic binder.
[0009] The ratio of the coal to the metallic oxide content may be practically
determined. For
.. example a stoichiometric ratio for the full reduction of iron in the ore
can be used.
[0010] The organic binder content may lie in the range of 0 to 1%. This
content may be
dictated by the physical properties of the resulting pellets principally the
strength of the
pellets in a green state and in an air-dried or indurated state. The pellets
may be may be air-
indurated for at least 4 days. This period is usually adequate to ensure that
the pellets are
sufficiently strong to allow their safe and efficient handling to subsequent
pre-reduction
reactors. The mechanical strength of the pellets is preferably above 600N, The
pellets
should also have an acceptable behaviour in a hot reactor environment to avoid
decrepitation
due to excessive swelling.
[0011] A single binder or a mixture of binders may be used. The invention is
not limited in
this respect.
CA 03005810 2018-05-17
WO 2017/087997 PCT/ZA2016/050049
3
[0012] Pre-reduced pellets are evaluated based on the reduction extent of iron
oxides
contained in the ore. Preferably the iron oxide should be present in a
quantity of less than
10% from the initial content. However a consistent pre-reduction yield should
be a main
target during a normal and stable operation.
[0013] The pellets may be subjected to a thermal reduction process or to a
hybrid, solid-
state, reduction process.
[0014] The pellets, air-dried and indurated, may be heated in a fixed bed
reactor at an
optimal residence time which may lie in a range of from 0.5 to 4 hours.
[0015] If a thermal pre-reduction step is adopted then the pellets may be
heated at a
temperature in the range of 1100 to 1200 C,
[0016] If the hybrid, solid-state, pre-reduction step is adopted then the
pellets may be heated
to a temperature in the range of 900 to 1000 C in a controlled atmosphere of a
reducing gas.
[0017] The reducing gas may comprise one or more of the following: CO, syngas
(CO + H2).
natural gas and hydrogen.
[0018] If a fixed bed reactor is employed then the reducing gas may be
filtered through a hot
burden in the reactor. The reducing gas flowrate should be selected to achieve
an adequate
reduction yield of the iron oxides in the ore, as well as acceptable reactor
operation
performance.
CA 03005810 2018-05-17
WO 2017/087997 PCT/ZA2016/050049
4
[0019] The invention finds particular application in the preparation of pre-
reduced, carbon-
based, ilmenite micro-pellets which are to be smelted e.g. in a DC open arc
furnace.
However, the principles of the invention may be employed for the pre-reduction
of pellets of
titaniferous magnetite, ferrochrome and ferromanganese ores for the subsequent
production
of titania slag, chrome and manganese, alloys respectively.
[0020] Reference has been made to heating the air-dried pellets in a fixed bed
reactor. This
is exemplary only and non-limiting. A moving bed and a rotary kiln may be
employed in place
of the fixed bed reactor, in a pre-reduction stage. It is important that
abrasion of the pellets is
minimised and it should be possible to separate pre-reduced fines from other
material, for
example through the use of magnetic or equivalent techniques.
BRIEF DESCRIPTION OF THE DRAWING
[0021] The invention is further described by way of example with reference to
the
accompanying figures wherein;
Figure 1 illustrates in flow chart form the pre-reduction of carbon-based,
ilmenite micro-
pellets and the subsequent smelting thereof;
Figure 2 is a diagram depicting an impact of the residence time on pre-
reduction and
metallisation degrees at 1000 C and 0.51 CO / min; and
Figure 3 is a diagram depicting an impact of the CO flowrate on the pre-
reduction and
metallisation degrees at 1000 C and lh residence time.
5
DESCRIPTION OF PREFERRED EMBODIMENT
[0022] The invention is hereinafter described with reference to the pre-
reduction of carbon-
based, ilmenite, micro-pellets. Although this is a preferred application of
the principles of the
invention it is possible to adapt the principles described herein for the pre-
reduction of
titaniferous magnetite, ferrochrome and ferromanganese ores.
[0023] Raw ilmenite ore 10 of a suitable size is fed to a blender 12. The
blender also receives
coal fines 14 of -106 micron in size and an organic binder 16 formed from a
single binder or
from a mixed binder composition.
[0024] The ratio of the input coal to the ilmenite is determined taking into
account practical
considerations. For instance a stoichiometric ratio which achieves a full
reduction of iron in the
ilmenite ore can be used. Further, the input of organic binder or mixes of
organic binders, in
the range of up to 1%, is dictated by the physical properties of the resulting
pellets, particularly
the green and air-dried strengths of the pellets. The resulting pellets should
also have an
acceptable behaviour (subsequently) in a hot reactor environment to avoid
decrepitation due
to excessive swelling.
[0025] The blender 12 produces carbon-based, ilmenite, micro-pellets 18 of 2mm
to 5mm in
size. These pellets 18 are then air-dried (step 20).
[0026] The air-dried, indurated pellets 21 are then subjected to a thermal pre-
reduction step
22, or to a hybrid, solid-state pre-reduction step 24. In each instance the
air-dried indurated
Date Recue/Date Received 2021-09-21
6
pellets are heated in a fixed bed reactor 26 for an optimal residence time,
generally from 0.5
to 4 hours.
[0027] If use is made of the thermal pre-reduction process the pellets are
heated in the reactor
26 to a temperature in the range of 1100 to 1200 C. If use is made of the
hybrid approach
then the pellets are heated in the reactor 26 to a temperature of 900 to 1000
C in a controlled
atmosphere of a reducing gas 30 which comprises one or more of CO, syngas,
natural gas
and hydrogen. The reducing gas is filtered through the hot burden of the
pellets in the reactor
26. The reducing gas flowrate is regulated to achieve an adequate pre-
reduction yield. The
flowrate should also be regulated to optimise the reactor operation,
principally the thermal
efficiency and the production cost.
[0028] Process parameters of importance, in respect of the of pre-reduction
technique used,
include: the ilmenite grain size distribution, the composition of the pellets,
the sizes of the
pellets, the operating temperature, the residence time and the reducing gas
flowrate.
[0029] Taken under consistent operating conditions each method is able to
produce a
consistent pre-reduction yield. The hybrid method, despite operating at a
lower temperature
then the thermal reduction method, appears to offer a higher pre-reduction
yield than the
thermal method.
[0030] The fully or partially pre-reduced ilmenite pellets 32, emerging from
the reactor 26, can
be fed, cold or hot, to a conventional ilmenite smelting process 36.
Date Recue/Date Received 2021-09-21
7
[0031] Without being bound by the following explanation it is believed that
the organic binder
provides a more intimate contact between the ilmenite and the coal fines. The
small pellet size
feature, in a highly reducing atmosphere, assists the transfer of heat and
mass in the diffusion
of gaseous reductants, such as CO and H2, to the reaction sites. The organic
binder 16 burns
off at the process temperature, a feature which induces localised reduction
and promotes the
formation of cracks and pores in the ilmenite ore grains contained in the
pellets 32. The specific
surface areas of the ilmenite pellets are therefore increased and the
diffusion rate of the gas
reductant to the reaction sites is enhanced. This in turn impacts on the pre-
reduction yield.
The reduction process can be smoothly and efficiently operated despite the
minor sintering of
the pellets that may occur at elevated temperatures.
[0032] The fully or partially pre-reduced, carbon-based ilmenite pellets which
are fed, either
hot or cold, into a DC open arc furnace decrease the consumption of
electricity in the furnace,
help to address slag foaming and result in an improved grade of TiO2 slag 38
output by the
furnace.
[0033] Through tests it has been established that iron oxide in the pellets
was nearly
completely reduced through the use of the hybrid pre-reduction process carried
out at a
temperature of 1000 C and for a residence time of 2 hours. The pre-reduction
yield was
increased as temperature, residence time and reducing gas flowrate were
increased.
[0034] The use of the thermal pre-reduction process at a temperature of 1100
to 1200 C
produced a pre-reduction yield of about 85% - a value which is adversely
affected with an
increase in ilmenite ore grain size and with an increase in the size of the
coal fines.
Date Recue/Date Received 2021-09-21
8
[0035] About 4 tons of cold pre-reduced ilmenite pellets were smelted in a DC
open arc
furnace. The energy consumption of the furnace lay in the range of 0.6 to 0.7
kWh / kg of pre-
reduced ilmenite pellets ¨ a figure which represents an electrical energy
saving of 30 to 40%
compared to a conventional ilmenite smelting process. The smelting process was
stable with
no visible sign of foaming. The product 38 contained about 95% TiO2 and about
3% FeO.
[0036] A higher grade TiO2 slag (above 90%) can thus be achieved, using
conventional
ilmenite feedstock in smelting operations, with no foaming occurring. Using
the method of the
current invention, a lower grade ilmenite could be used as feedstock to
produce TiO2 slag of at
least 85% TiO2 content.
[0037] The invention has been described with reference to the use of a gaseous
reductant. It
is possible though to make use of a solid reductant such as anthracite or
coal, instead of the
reducing gas 30. Also the reactor 26 which, typically, is a fixed bed reactor
can be replaced
by a moving bed or by a rotary kiln configuration provided abrasion effects
between the pellets
are minimised. It should be possible though to separate the pre-reduced
pellets, for example
using magnetic techniques, from the other material emerging from the reactor.
[0038] Additional carbonaceous solid reductant can be used in excess to reduce
residual iron
in the slag to below 6% without inducing slag foaming.
ACTIVE_CA\ 47565485\1
Date Recue/Date Received 2021-09-21
CA 03005810 2018-05-17
WO 2017/087997 PCT/ZA2016/050049
9
DESCRIPTION OF A PILOT TEST OF THE PROCESS OF THE INVENTION
[0039] A 200 kW DC arc furnace facility was used for demonstrating the
smelting of pre-
reduced ilmenite pellets. The furnace had a 1 m outer diameter, water-spray
cooled steel
shell lined with a single layer and three rows of magnesite-chrome bricks and
a hearth lined
with rammable magnesia. The refractory lining resulted in the furnace crucible
internal
diameter (ID) of 0.656 m. The furnace was equipped with an alumina lined
conical roof and a
shell bolted on a domed base. A single taphole was used to tap a stream of
both molten slag
and metal. The furnace was equipped with a single and centrally-located
graphite electrode
of 40 mm diameter operating as a cathode while the anode comprised steel pins
buried in the
hearth. The feed system comprised individual hoppers used to feed anthracite
and pre-
reduced ilmenite pellets through a furnace feed pipe. The furnace was equipped
with an off-
gas system for the cleaning of produced process gas prior to release thereof
into the
atmosphere.
[0040] Carbon-based ilmenite pellets containing the as received ilmenite,
stoichiometric
amount of anthracite, were prepared using a proprietary organic binder at a
required dosage.
The as-received ilmenite had a particle size distribution of D100 in the 38 pm
to 150 pm size
range. The anthracite was milled to a D85 passing 106 pm to facilitate its
incorporation into an
ilmenite pellet recipe. Pellets were prepared in a pilot-scale pelletizing
unit comprising an
inclined rotating pan of 985 mm diameter and 170 mm depth. The mechanical
strengths of
.. the pellets were measured and found to vary with the type and dosage of
binder used. within
a range of 0.01 - 0.03 MPa for green pellets and 0.81 - 1.50 MPa for indurated
pellets at
ambient conditions.
CA 03005810 2018-05-17
WO 2017/087997 PCT/ZA2016/050049
[0041] Batches of 250 kg each of indurated pellets were reduced in an
electrically heated
muffle furnace operated at a controlled temperature of 1100 C. During a three
hour firing
time, in total 5 kg of CO was blown intermittently through the reactor burden
at intervals of 10
minutes. The pellets were loaded in a single tray of 1700 mm x 900 mm, having
a loading
5 area of a mesh screen acting as a distribution plenum for the reducing
gas.
[0042] Both the raw and pre-reduced ilmenite materials at various conditions
were
chemically analysed; specifically an analysis of the iron oxidation states
(Fe3+, Fe2', and Fe )
was used to calculate the pre-reduction and metallisation yields for the
pellets. Negligible
reduction of titanium oxides was assumed throughout and the pre-reduction
yield was
10 therefore calculated based on the mass balance of oxygen associated with
each gram of iron
before and after pre-reduction. Equations [1] and [2] were used for the
calculation of the
prereduction and metallization yields, respectively.
(Oxygen removed by the pre¨reduction process)
Pre¨reduction yield =100x
(Oxygen associated with each gram of iron in the ilmenite sample) .. [1]
eu
Metallisation yield =100x F .................................... . [2]
[0043] The chemical analyses of the ore and anthracite are summarised in Table
1 and
Table 2, respectively.
CA 03005810 2018-05-17
WO 2017/087997 PCT/ZA2016/050049
11
Table 1: Bulk chemical composition of the raw ilmenite (mass %)
MgO A1203 SiO2 CaO TiO2 V205 Cr2O3 MnO FeO Fe2+ Fe Fe/Ti
0.28 0.44 0.57 0.05 46.6 0.25 0.09 1.08 47.87 13.50 <0.05 1.33
<0.05%: the analyte concentration could not be accurately quantified as it is
below the limit of detection (LOD)
Total Fe in the sample is expressed as % Fe0
Table 2: Summary of the bulk chemical composition of the anthracite (mass %)
Moisture Ash Volatile Fixed Total Total
carbon carbon sulphur
3.57 4.74 6.19 89.1 90.7 0.56
_
[0044] In total, about 3.6 tons of pre-reduced pellets were produced. The
pellets were
bagged in 1 m3 bags from which five composite samples were collected. The
chemical
analyses of the 5 composite samples are given in Table 3.
Table 3: Chemical compositions of the pre-reduced pellets
1 Total I 1 _
Fe'T1
I MgO A1203 S102 Ca0 TiO2 V205 Cr2O3 MnO Fe Fe Fe2+ C
ratlo
TP Bag 7.23
1 0.53 0.33 0.31 0.10 44.4 0.36 0.07 1.05 34.92 25.55 9.37
1 6.55 1 32
TP Bag i 7.72
2 0.55 0.30 0.26 0.07 44.5 0.36 0.08 ; 1.06 35.23
25.44 9.79 1 6.76 1.32
TP Bag 7.97
3 0.50 . 0.30 0.24 0.15 43.5 0.33 0.07 1.03
33.60 21.90 11.7 1 1 5.45 1.29
TP Bag ; 8.01
4 0.48 0.32 0.39 0.11 42.4 0.33 0.07 1.05 34.46 22.66 11.8 1
4.78 1.35
TP Bag 7.20
5 0.36 0.45 0.64 0.18 41.9 0.33 0.07 1.08 36.01 24.21
11.8 4.66 1.43 _1
Average 0.48 0.34 0.37 0.12 43.3 0.34 0.07 1.05
34.84 23.95 10.89 7.635.64 f 1.32 _
0.39
St dev 0.07 0.06 0.16 0.04 1.17 0.02 0.004 0.02
0.90 1.64 1.21 0.98 0.02
CA 03005810 2018-05-17
WO 2017/087997 PCT/ZA2016/050049
12
[0045] The calculated degrees of prereduction and metallization for the five
composite
samples are presented in Table 4
Table 4: Pre-reduction and metal//sat/on degrees of ilmenite pellets
Composite sample no. Pre-reduction degree, % Metallisation degree,
%
1 79.7 73.2
2 78.9 72.2
3 73.6 65.2
4 74.1 65.8
75.1 67.2
Average 76.3 68.7
St dev 2.8 3.7
5 .. [0046] Tables 3 and 4 show that pellets prereduced to a consistent extent
were produced as
a result of the uniform furnace operating conditions.
[0047] Results from laboratory tests in a tube reactor of 80 mm diameter
showed a very
important feature of this process that is presented in Figures 2 and 3. Tests
conducted at a
temperature of 1000 C, showed that pre-reduction and metallisation degrees are
both related
.. to the residence time and CO flowrate. Increasing the CO flowrate appears
to positively affect
the yields, suggesting that CO diffusion would play a significant role in this
process.
CA 03005810 2018-05-17
WO 2017/087997
PCT/ZA2016/050049
13
[0048] Continuous smelting of partially reduced ilmenite pellets (approx.. 70%
prereduction
yield) was carried out to demonstrate stable furnace operation as well as
production of a
consistent slag quality, in particular, a slag TiO2 grade above 85%. The test
work also had
the objective of confirming the process specific energy requirement. The slag
results are
presented in Table 5.
Table 5: Analysis of slag from the stable smelting operation, in mass %
i 1
Tap .1102 Cr203 MnO FeO V205 S102 Ca() 1 A1203
1 M90
A 84.74 1,34 1,37 2,06 0,30 1,31 0,34 1,96 6.58
_
B 87.84 0,60 1,18 2,59 0,21 1,16 0,28 i 2,49
3,65 _
C 83.15 0,31 1,68 2,73 0129 1,20 0,38 ' 2,76
7,50
D 88.86 0,45 0,68 4,89 0,17 0,49 0,31 1,45 2.70
E 88.19 0,45 0,99 4,37 0.16 0,43 0,35
1,53 , 3.53
F 91.27 0,31 1,47 1,58 0,15 0,28 0,32 1,47 3,15
G 94.28 0,08 1,32 1,25 : 0,10 0,21 0,17 0,98 .
1,61
H 93.32 0,09 1,21 1,48 0,10 0,16 0,14 1,10 :
2,40
*by difference
Table 6: Evolution of composition of pig iron from the stable smelting
operation, in mass %
Tap Fe Ti V Si Cr Mn C P S
A 95.54 0.67 0.24 0.34 1.04 0.48 1.66 0.02 0.01
B 95.92 0.43 0.23 0.36 1.08 0.44 1.53 0.00 0.01
C 95.15 0.55 0.16 0.38 1.47 0.33 1.92 0.02 0.02
D 97.03 0.16 , 0.09 0.18 0.52 0.21 1.76 0.02 0.03
E 94.59 0.29 0.20 0.62 1.14 0.51 2.57 0.03 0.05
F 91.81 3.64 0.23 0.64 1.17 0.86 1.62 0.01 0.02
G 95.83 0.57 0.20 0.41 0.71 0.60 1.64 0.02 0.02
H 93.66 1.34 0.32 1.28 0.89 0.88 1.56 0.02 0.05
[0049] Slag FeO contents as low as 1.3 % were achieved without visible signs
of slag
foaming. This condition was maintained for a longer period during which stable
furnace
operation was demonstrated and slags of consistent FeO content were produced.
Results for
CA 03005810 2018-05-17
WO 2017/087997 PCT/ZA2016/050049
14
this particular test work suggest that smelting of partially reduced ilmenite
and operating the
furnace with lower FeO content in the slag are technically possible.
[0050] The 200 kW DC open-arc furnace was operated at a power level in the
range of 115
¨ 140 kW and at a corresponding voltage of 100¨ 115 V. Consistent furnace heat
losses in
the range of 60 ¨ 90 kW were measured. Average tapping temperatures measured
using an
optical pyrometer were scattered within a range between 1670 and 1780 C. The
specific
energy requirement (SER) for the smelting of prereduced carbon-based pellets
was
measured between 0.6 and 0.7 kWh I kg prereduced ilmenite. A 30-40 % reduction
in furnace
electricity required relative to a conventional smelting process can be
achieved assuming that
a prereduction yield of at least 70 % can be achieved. Arc resistivities were
measured for
various conditions investigated in order to predict the furnace arc stability.
Arc resistivity was
found to be in the range of 0.0168 and 0.0240 D.cm which range is close to
0.0175 D.cm, a
typical value for arc resistivity in smelting processes with CO-rich
atmospheres (in the
absence of foaming).
